Add comprehensive AppImage metadata extraction, display, and bug fixes

- Add AppStream XML parser (quick-xml) to extract rich metadata from bundled
  metainfo/appdata files: description, developer, license, URLs, keywords,
  categories, content rating, release history, and MIME types
- Database migration v9: 16 new columns for extended metadata storage
- Extended inspector to parse AppStream XML, desktop entry extended fields,
  and detect binary signatures without executing AppImages
- Redesigned detail view overview tab with 8 conditional groups: About,
  Description, Links, Release History, Usage, Capabilities, File Info
- Fix crash on exit caused by stale GLib SourceId removal in debounce timers
- Fix wayland.rs executing AppImages directly to detect squashfs offset,
  replaced with safe binary scan via find_squashfs_offset_for()
- Fix scan skipping re-analysis of apps missing new metadata fields

src/core/integrator.rs

@@ -269,6 +269,22 @@ mod tests {
             tags: None,
             pinned: false,
             avg_startup_ms: None,
+            appstream_id: None,
+            appstream_description: None,
+            generic_name: None,
+            license: None,
+            homepage_url: None,
+            bugtracker_url: None,
+            donation_url: None,
+            help_url: None,
+            vcs_url: None,
+            keywords: None,
+            mime_types: None,
+            content_rating: None,
+            project_group: None,
+            release_history: None,
+            desktop_actions: None,
+            has_signature: false,
         };
 
         // We can't easily test the full integrate() without mocking dirs,